1. Field of the Invention
The present invention generally relates to communication systems, and more particularly to a system and method for controlling a mobile terminal in a communication system when, for example, it includes W-CDMA (Wideband Code Division Multiple Access) systems.
2. Background of the Related Art
Generally, in a mobile communication system, a base station should receive the same level of wireless power output from a plurality of mobile terminals. This helps to maintain a channel capacity fixedly allocated to the base station.
When the output power of one of the terminals is higher, an adjacent mobile terminal regards the output power of the first mobile terminal as noise and thus increases its own output power in order to secure a stable signal-to-interference ratio (SIR).
In addition, if there is noise caused by a bad wireless link state, mobile terminals will raise their wireless transmission power or output power. Because the adjacent terminal also performs the same operation, the situation will develop where the plurality of mobile terminals will competitively raise their own output powers, and this will cause many problems. For example, the fixed channel capacity of the base station may be consumed, the channel capacity for communications may be reduced, the reliability of the system will deteriorate, and a load burden to the system will increase.
In order to solve such problems in the W-CDMA mobile communication system, first, each mobile terminal will monitor the state of a radio channel (that is, radio link) periodically. If the radio channel is not in good condition, the terminal shuts off its output power, and then when the radio channel gets back to good condition the terminal will turn its output power back on.
FIG. 1 illustrates a general W-CDMA system, which includes a user equipment (UE) 10 for providing mobile communications to a user, a base station 20 for serving as a wireless interface for the UE, and a switching center 30 connected by wire to the base station for switching a call.
The W-CDMA system operates as follows. When the UE, that is, a mobile communication terminal, pages an opposite party desired for communication while being moved within a service region formed by the base station, the wirelessly connected base station transmits a corresponding signal to the switching center. The switching center 30 then analyzes the received signal and sets a communication path through switching, and accordingly the mobile terminal 10 communicates with the desired party. The wireless communication path, that is, the radio channel, formed between the terminal and base station can transmit and receive video as well as voice and data.
FIG. 2 illustrates a downlink frame used in the W-CDMA system. The frame has a 10 ms period and includes a plurality of slots (slot#0˜slot#15). One slot includes a first data field, a TPC (Transmit Power Control) field, a TFCI (Transmit format Combination Indicator) field, a second data field, and a pilot field. The first and second data fields constitute a data channel (that is, a DPDCH (Dedicated Physical Data Channel), and the TPC, TFCI, and pilot fields constitute a control channel (that is, a DPCCH (Dedicated Physical Control Channel).
Each slot has 2560 chips. The TPC field is used for designating an output power control function. The TFCI field indicates a construction of a frame format. And, pilot field provides information for measuring a quality of the DPCCH to obtain frame synchronization and control ON/OFF of output power of the mobile communication terminal.
Generally, in the W-CDMA system, in order to decode a current radio channel, a mobile terminal needs to obtain synchronization of the radio channel. In addition, the mobile terminal must periodically report a synchronization state of the radio channel or a quality of the radio channel to an upper layer. In determining the frame synchronization state using a pilot channel (pilot field), the mobile terminal uses quality measurement of a CRC (Cyclic Redundancy Check) and DPCCH. The mobile communication terminal especially performs the frame synchronization by using the pilot channel (pilot field) of the DPCCH.
Also, in the W-CDMA system, ON/OFF of output power is controlled according to a quality of the DPCCH, and in this case two threshold values are set to control ON/OFF of output power of the mobile terminal. The first threshold value is used for turning OFF output power and the second threshold value is used for turning ON output power.
A W-CDMA system standard known as 3GPP TS 25.101 proposes to use the two threshold values to determine a quality of the control channel as follows: the first threshold value (Qout) is to be set with a 30% downlink TPC command error ratio level and the second threshold value (Qin) is to be set with a 20% downlink TPC command error ratio level. This standard, however, only proposes conditions for determining threshold values for controlling ON/OFF of output power of a mobile communication terminal, does not mention a method for determining the threshold values themselves. A need therefore exists for a method of determining the threshold values for controlling ON/OFF of output power of a mobile terminal in a W-CDMA system.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.